The present invention comprises apparatus for measuring the drainage time of a papermaking stock suspension in a manner that can accurately predict the performance of the stock on the forming section at the wed end of a paper machine.
Paper makers have long had available a choice of simple methods and apparatus for estimating drainage time of a given papermaking stock. One method (TAPPI Standard Drain Time) normally involves placing a standardized sample of the stock in a small laboratory sheet mold and visually estimating the time for sheet formation when a drain valve is opened. The values so obtained may or may not correlate well with the performance of the same stock at the paper machine wet end.
A more sophisticated way of measuring the drainage characteristics of a papermaking stock is given in U.S. Pat. No. 4,613,406 to Gess. In this patent a slurry is dewatered in a sheet mold using vacuum under the forming wire. The pressure differential across the sheet is measured as a function of time. When the data so obtained are plotted, the resulting curve shows four generally linear sections of different slopes. The inflection point between the first two linear curves is believed to note the transition point of a random collection of fibers forming a web as the stock is being dewatered. This inflection point is believed to correlate with the wet line on the forming section. The inflection point between the second and third linear sections indicates where there is no further compaction of the web from the forces resulting from the vacuum applied near the end of the forming section. This is believed to correspond closely to the dry line on a forming section. Finally, the inflection point between the third and fourth linear portions of the curve indicates where dewatering is essentially complete and air is first drawn through the web. The apparatus and method of its use is further described by the above inventor in the following publications: Proceedings, TAPPI Papermaking Conference, Portland, Oreg., pp. 185-189 (1983) and Notes, TAPPI Retention and Drainage Seminar, pp. 75-81 (1983).
The above author further describes the use of the above drainage time measurement apparatus in TAPPI, Advanced Topics in West End Chemistry Short Course, Memphis, Tenn. (1987). In this paper he additionally shows and describes a curve of sheet weight versus drainage time and discusses how this relates to fines sensitivity of the forming system. A curve having two linear portions of different slope results as the sheet weight is increased. This is further discussed by the same author in two more recent papers: TAPPI, Retention and Drainage Short Course, pp. 49-52 (1989) and Preprints, TAPPI Annual Meeting, pp. A27-A32 (1989).
The first linear portion of the plot of sheet weight versus drainage time indicates that forming wire geometry is of primary importance in the retention of fine particles. The second linear portion suggests that the sheet itself is acting as a filter medium for fines retention.
While the apparatus described in U.S. Pat. No. 4,613,406 has seen widespread use, it is basically a laboratory instrument rather than one which can be used in the hour-to-hour control of an operating paper machine. The present invention provides apparatus which can rapidly form a series of sheets of differing basis weight to give the wet end tender a timely indication of factors such as white water fines buildup that would affect operation of the forming section.